This invention relates to a filtering apparatus of the type utilizing elongate baglike tubular filters for filtering particulate material which is entrained in a gas stream. A filtering apparatus of this type generally includes a multiplicity of tubular filter bags mounted in a filter housing or "baghouse" with each filter bag having a supporting frame or "cage" positioned therein so as to hold the filter bag in an open hollow tubular configuration. Particulate-laden gas is directed into the filter housing and flows through the gas permeable filter bags while the particulate material is filtered and retained on the exterior surface of the filter bags. Periodically, a reverse purge of air is directed into the outlet end of the tubular filter bags for dislodging the trapped particulate material from the filter bags and thus cleaning the filter bags.
One of the problems with this type of filtering apparatus is that the cleaning operation is not very effective in removing the particulate material from the filter bags. After the filter bags have been in use for a period of time they become increasingly clogged with particulate material. This undesirably increases the pressure drop across the filtering apparatus and significantly reduces its efficiency. In some filtering applications, the inability to effectively clean the filter bags of the particulate material is so severe that it is necessary to frequently replace the filter bags.
Also, as a result of the particulate material which is trapped in the filter bags and the resulting obstruction to air flow, the filtering apparatus must be designed with a relatively large number of filter bags and with a relatively low ratio of air flow to filter area in order that the filtering apparatus will remain serviceable as the filter bags become increasingly clogged with particulate material.
The presence of particulate material in the filter bags also causes abrasive wear of the bags and results in a shortened useful life. Typically, a filter bag will have a useful life of up to about two years. However, in severe applications, as for example where the particulate material is of a highly abrasive nature, the filter bags may wear out in a matter of weeks. Considering that the usual filtering apparatus may contain many hundreds of filter bags, and that a single bag may cost from fifty to one hundred dollars, the cost of continually replacing the filter bags makes the operational cost of the filtering apparatus quite high. Yet, in many applications the use of filtration apparatus is required by governmental air quality standards in order to limit the amount of particulate material in effluent gases.
Until the present invention, there has been no satisfactory solution to the problems of excessive filter bag wear and ineffective cleaning of the filter bags in a bag-type filtering apparatus. While various attempts have been made to improve the cleaning of the filter bags, as disclosed in U.S. Pat. Nos. 3,053,030 and 3,874,857, for example, the prior attempts have not adequately solved the existing problems.